Two-layer golf balls are typically made with a single solid core encased by a cover. These balls are generally most popular among recreational golfers, because they are durable and provide maximum distance. Typically, the solid core is made of polybutadiene crosslinked with zinc diacrylate and/or similar crosslinking agents. The cover material is a tough, cut-proof blend of one or more materials known as ionomers, such as SURLYN®, sold commercially by DuPont, FORMION®, sold commercially by A. Schulman, Inc., or PRIMACOR® or DOWLEX®, both sold commercially by Dow Chemical Co.
Other multi-layer golf balls may have multiple core layers, multiple intermediate layers, and/or multiple cover layers. They tend to overcome some of the undesirable features of conventional two-layer balls, such as hard feel and less control, while maintaining the positive attributes, such as increased initial velocity and distance. However, it is desirable that multi-layer balls have a “feel” similar to wound balls, especially for more advanced players.
Additionally, the spin rates of golf balls affect the overall control of the balls depending on the skill level of the players. Golf balls with lower spin rates exhibit improved distance, but are harder to control on short shots, such as approaches to greens. Conversely, higher spin rates afford skilled players more control, but inhibit driving distances. To strike a balance between the spin rates and the playing characteristics of golf balls, additional layers, such as intermediate layers, outer core layers and inner cover layers, are often added to the solid core golf balls to improve the playing characteristics of the ball.
The patent literature discloses a number of multi-layer golf balls. U.S. Patent Pub. No. 2005-0130767 A1, which is commonly owned and incorporated herein by reference in its entirety, is directed to an improved multi-layer golf ball displaying a certain spin profile. The ball has a generally rigid, thermosetting polybutadiene outer core surrounding a relatively soft, low compression inner core. The inner core has a hardness that is less than the hardness of the outer core, and a specific gravity that is less than or equal to the specific gravity of the outer core. The inner core and outer core are formulated to provide a combined overall core compression of greater than about 50.
U.S. Pat. No. 6,685,579, which is commonly owned and incorporated by reference in its entirety, is directed to golf balls having a cover comprising three or more layers: an inner cover layer, an outer cover layer, and an intermediate cover layer. The outer cover layer comprises a composition formed of a reactive liquid material, and the combination of the thickness of the cover layers is about 0.125 inches. Golf balls prepared accordingly can exhibit substantially the same or higher coefficient of restitution (“COR”), with a decrease in compression or flexural modulus, compared to golf balls of conventional construction. The resultant golf balls typically have a COR of greater than about 0.7 and an Atti compression of at least about 40. As used herein, the term coefficient of restitution for golf balls is defined as the ratio of the rebound velocity to the inbound velocity when balls are fired into a rigid plate. A discussion of COR and suitable test methods for measuring COR can be found, for example, in U.S. Pat. No. 6,547,677, which is incorporated herein by reference.
U.S. Patent Pub. No. 2004-0082407 A1, which is also commonly owned and incorporated by reference in its entirety, is directed to a golf ball comprising an inner core, an outer core, and a cover. At least one layer of the golf ball is made from a low compression, high COR material, and is being supported by a low deformation, high compression layer. The resulting golf ball has high COR at fast and slow impact speeds and low compression for controlled greenside play.
Varying materials, density, or specific gravity among the various layers of a golf ball controls the spin rate of the golf ball. For instance, redistributing weight from the outer layers of the golf ball to the inner layers decreases the moment of inertia of the golf ball, thereby increasing the spin rate and vice versa as discussed in commonly owned U.S. Pat. No. 6,494,795 B2.
Hence, there remains a need for multi-layer golf balls with improved distance and feel for low swing-speed players.